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The Photosynthetic Response of Poplar Trees to CO2 Enrichment from the Seedling Stage to Maturity
Reference
Bernacchi, C.J., Calfapietra, C., Davey, P.A., Wittig, V.E., Scarascia-Mugnozza, G.E., Raines, C.A. and Long, S.P. 2003. Photosynthesis and stomatal conductance responses of poplars to free-air CO2 enrichment (PopFACE) during the first growth cycle and immediately following coppice. New Phytologist 159: 609-621.

Background
Most CO2 enrichment studies of trees are conducted on juvenile individuals; yet, as the authors of this unique study note, "most terrestrial carbon assimilation is by mature trees in closed canopies." To bridge this gap, they worked with hybrid poplars, which they describe as "so fast growing that they provide a rare opportunity to grow a plantation forest from planting to canopy closure of tall trees (greater than 9 m) in just 3 years."

What was done
The work was conducted in the Poplar Free Air CO2 Enrichment (PopFACE) experiment described by Miglietta et al. (2001), which is located near Viterbo in Central Italy, where a field previously used for wheat cultivation was planted with the hybrid Populus x euramericana Dode (Guinier) (P. deltoides Bart. ex Marsh. x P. nigra L., I-214), with the exception of six 30 x 30-m square plots that each contained a 22-m-diameter FACE ring. Three of these rings were maintained at 370 ppm CO2, while the other three were maintained at 550 ppm CO2. Within each ring were grown equal-area sections of P. alba L. (genotype 2AS1), P. nigra L. (genotype Jean Pourtet) and P. x euramericana (genotype I-214), which were maintained free of drought by a drip irrigation system, while periodic measurements of net photosynthesis and stomatal conductance were made over the three-year period of growth from the seedling to closed-canopy forest stage.

What was learned
There was no response of leaf stomatal conductance to atmospheric CO2 enrichment. In the case of net photosynthesis, however, the team of seven scientists observed a 38% increase in light-saturated net photosynthesis at 25C, which they describe as being "close to the maximum theoretically possible" in response to the 49% increase in atmospheric CO2 concentration employed in their study. Daily integrated rates of in situ photosynthesis were even higher, rising by 40 to almost 90% (which is approximately equivalent to 150% in response to a 300-ppm increase in the air's CO2 concentration), due to the fact, in the words of Bernacchi et al., that "daytime leaf temperatures were typically over 30C resulting in a larger stimulation of leaf photosynthesis by elevated CO2 than would be evident at 25C (Long, 1991)."

What it means
The huge stimulation of daily net photosynthesis demonstrated in this unique real-world study illustrates the enormous potential for earth's trees, even in closed-canopy forests, to positively respond to the ongoing rise in the air's CO2 content. Few people in the past, particularly climate alarmists, have been willing to acknowledge anything near what is revealed in these results.

References
Long, S.P. 1991. Modification of the response of photosynthetic productivity to rising temperature by atmospheric CO2 concentrations: Has its importance been underestimated? Plant, Cell and Environment 14: 729-739.

Miglietta, F., Peressotti, A., Vaccari, F.P., Zaldei, A., deAngelis, P. and Scarascia-Mugnozza, G. 2001. Free-air CO2 enrichment (FACE) of a poplar plantation: The POPFACE fumigation system. New Phytologist 150: 465-476.


Reviewed 16 June 2004